Caps have been used for many years, for example, on automobile radiators to enable the operator to replenish the radiator fluid. Generally speaking, the radiator caps comprise a screw or twist on/off type cover to provide a substantially watertight closure of the radiator and for retaining under pressure the fluid within the radiator. Oftentimes, however, the fluid within the radiator becomes superheated which hitherto has presented a serious risk of burn injury to those attempting to remove the radiator cap.